1. Field of the Invention
The present invention relates to computer graphics systems, and more specifically to a method and apparatus for scanning a digital display screen of a computer screen at a horizontal scanning frequency lower than the origin frequency of a display signal.
2. Related Art
Digital display units are often used in computer systems to display images. Typically, an image is sent to a digital display unit encoded in the form of a display signal (e.g., RGB analog signals or PanelLink digital signal) and the display unit reproduces the image represented by the display signal. Digital display units are characterized by discrete points (referred to as "pixels") on a display screen, and these points are typically activated individually based on the received display signal. An image is produced as a result of such collective actuation of the pixels.
When encoding an image in a display signal, a graphics source may encode display data at a given frequency (hereafter "origin frequency"). For example, in an SVGA compatible environment well known in the art, an image may be first represented as discrete pixel data elements at a graphics source, and an analog display signal may be generated from these pixel data elements at what is commonly known as a dot clock frequency. The analog display signal encoded with the display data at the origin frequency is transmitted to a digital display unit for display on a digital display screen.
There is a tendency in the industry to provide display signals at higher origin frequencies as the higher frequencies enable higher refresh rates (the number of images scanned/displayed per second on a display screen) and finer resolutions. High refresh rates generally lead to images of better display quality (e.g., reduced flicker) and finer resolutions can provide for more detail in the displayed images as is well known in the art. As an illustration, the origin frequency in an SVGA compatible environment is computed by multiplying the desired refresh rate, the number of horizontal lines in each frame, and the total number of samples in each horizontal line. Some earlier systems generated display signals at an origin frequency of as low as 110 MHZ, while the present systems can generate display signal at an origin frequency of 250 MHZ.
A digital display unit receives a display signal with the encoded image, samples the received display signal to generate sampled pixel data elements, and actuates individual pixels of a digital display screen based on the sampled pixel data elements to generate an image. In general, it is desirable that the encoded signal be sampled at a sampling frequency equal to origin frequency for a proper reproduction of an image encoded in the display signal as is well known in the art.
Further, the digital display screen may need to be refreshed with each image encoded in each frame of the received signal. Accordingly, some prior systems may refresh the display screen using a horizontal scanning frequency (hereafter `scanning frequency`) equal to the sampling frequency, at least in situations when upscaling is not required.
However, it may be desirable to scan a digital display screen at a scanning frequency lower than the origin frequency of the received display signal. For example, some digital display screens may not be implemented to operate at horizontal scanning frequencies as high as the sampling frequency (or origin frequency) of the received display signals. A digital display may not be able to operate at high scanning frequencies as the electronic circuitry which actuates individual pixels on a digital display screen may not be designed (or otherwise be incapable of operating) for scanning frequencies as high as a corresponding sampling clock frequency. In such situations, it may not be possible to display the images encoded in the received display signals. Such inability to display images may not be unacceptable.
It may be desirable to scan digital display screens at lower frequencies for other reasons as well. For example, in some power constrained environments such as notebook computers, it may be desirable to scan digital display screens at lower horizontal frequency as such slower speed of operations generally consumes less electrical power.
Thus, what is needed is a method and apparatus for scanning a digital display screen of a computer screen at a horizontal scanning frequency lower than the origin frequency of a received display signal.